Treatment of nitrocellulose dope



Patented Feb. 13, 1945 n. ra W UNITED STATES PATENT OFFICE 2,369,200 W TREATMENT OF NITROCELLULOSfiDOPE Linton C. Amber-son, Edenburg, a.

No Drawing. Application September 6, 1941, Serial No. 409,836

3 Claims.

My present invention pertains to the treatment of nitrocellulose dope and is more particularly concerned with procedure for rendering such dope non-inflammable.

Ni roce os"d'6'pb,"which is a viscous solution of nitrocellulose in a suitable Volatile organic solvent, has been extensively employed for coat;

,,ingtheiabric-oncloth used for parts of certain types of airplanes and on balloons and other aeronautical equipment. It has been and is being utilized on parts of training planes and other planes which are partially non-metallic to cover the wings, ailerons, tail, elevators, rudders and fins and the like.

It is well known that nitrocellulose dope is highly inflammable and extremely objectionable from that point of view, even though it ha highly desirable qualities from the standpoint of forming a tough, useful skin or film having value for aeronautical purposes. Despite its inflammability, nitrocellulose dope has been widely used and is still used on a number of types of planes, particularly small privately owned planes, trainer planes, etc.

Efforts have been made in the past to reduce the infiammability of such dope and, while some progress has been made, the problem has by no means been satisfactorily solved. Some of the prior efforts to reduce the inflammability of nitrocellulose dope have caused additional problems to be raised, such as sweating, retarded or incomplete drying, loss of optimum strength and flexibility and similar difliculties. These prior attempts have thus not been widely adopted by the aeronautical industry,

One of the objects of my present invention is to provide a procedure by means of which nitrocellulose dope may be rendered non-inflammable at a very moderate cost without giving rise to other diificulties.

Another object of the invention resides in treating nitrocellulose dope so as to render it substantially or entirely non-inflammable and of such nature that, when applied to airplanes, contact with fire or flame at one point will not only not set fire to the dope but will prevent destruction of the lane by fire.

A further object of the invention resides in dissolving zinc or zinc oxide in hydrochloric acid to form zinc chloride and then mixing the nitrocellulose dope therewith and subsequently removing the zinc chloride and adjusting the consistency of the modified dope.

Other and further objects and advantages will be understood by those skilled in this art or will be apparent or pointed out hereinafter.

Nitrocellulose dope consists of a solution of nitrocellulose in a suitable volatile organic solvent. The nitrocellulose, also termed cellulose nitrate, is made in known manner by treating cellulose, such as cotton, with nitric acid under known conditions and dissolving it in a volatile solvent. The nitrocellulose dope, when applied to a surface, particularly a fabric or textile surface, forms a thin, tenacious, adherent skin or film thereon due to a rapid drying out through volatilization of the solvent. The skin or film is, however, of an inflammable nature irrespective of the solvent and there is generally some residual solvent present which increases the inflammability. Should any portion of such a skin or film be subjected to inflammable conditions, the entire surface will be rapidly destroyed,

It is important for various reasons to render the skin or film non-inflammable or at least highly resistant to burning so that, for example, if one portion of the treated surface were subjected to contact with fire it would be highly resistant thereto and would at the same time prevent any extension of the area which might be damaged to other portions of the surface.

In carrying out my present invention I place commercial strength (28-32%) hydrochloric acid (also known as muriatic acid) in a suitable container and introduce thereinto metallic zinc. The zinc reacts chemically with the hydrochloric acid, forming zinc chloride and giving oif gaseous hydrogen in accordance with the following reaction:

I prefer to add enough zinc to obtain complete reaction with the acid so that at the end of the reaction there is substantially no free hydrochloric acid present. I have found that this can be accomplished by employing acid and zinc in the relative proportions of approximately 2 pounds of zinc to 1 gallon of commercial strength acid. The zinc is preferably employed in the form of pig zinc and may be introduced into the acid either all at one time or incrementally as the reaction proceeds. I have further found that by subdividing the metallic zinc the reaction can be speeded up somewhat and, therefore, I deem it a part of my invention to break up the zinc into any size of pieces or particles which may be desired. In general, the smaller the pieces or particles of zinc, the more surface is exposed to the acid and the proportionately more rapid is the reaction. The exact proportions of zinc and acid to produce a complete reaction without any substantial excess of acid can be readily determined from the reaction set forth above, and from the same it can be calculated that approximately 65.4 parts of zinc interact with approximately 73 parts of hydrochloric acid based upon 100% acid. The necessary allowance must be made for the water present in the acid, During the reaction considerable heat is generated and the entire batch of material becomes hot. After the reaction has terminated, and this can be determined by observing when the evolution of hydrogen has ceased, I allow the batch to stand until it cools to about room temperature.

It will be understood that a suitable container is employed which is not. attacked by the acid or by the products of reaction. I have found, for example, that earthenware, preferably interiorly glazed, or glass forms a suitable material for the container, although this is not intended as restrictive because enameled or other special or treated metals can equally well be employed. The container should be of sufiicient size to make up a batch of the desired volume. The hydrogen which is evolved during the reaction may be recovered in any known or suitable manner for any desired purpose or it may be allowed to escape to the atmosphere. After interaction of the acid and zinc the resultant material is filtered and then transferred to another similar container which is provided with a valved outlet in its bottom which can be turned on or off at will for a purpose to be explained hereinafter. The valved outlet is made of a metal or alloy which is resistant to the action of chlorides such as the special irons known in the trade, stainless steel, and the like.

To the zinc chloride thus produced and after its transfer I add nitrocellulose dope in the proportions of approximately gallon of the dope to the amount of zinc chloride produced by interaction of about 1 gallon of acid and about 2 pounds of zinc, it being understood that where zinc oxide is employed instead of zinc enough zinc oxide is employed to provide the same amount of metallic zinc per gallon of acid. In this case the reaction will be:

Having added the nitrocellulose dope to the chloride in the container in the proportions indicated, I then thoroughly agitate and mix the same. This may be accomplished manually or by a mechanical stirrer and the agitating is continued for a long enough period to ensure that all of the dope has been contacted with chloride. It will be understood that the stirrer or agitator which is employed is of a material not attacked by the mixture.

Subsequent to the agitation the batch is allowed to stratify. This is accomplished by discontinuing the mixing or agitating and by allowing the batch to stand for a sufficient period of time until the treated dope rises to the top and the chloride settles at the bottom. Where a transparent container is employed, such as glass, the progress of the Stratification can be readily observed, and in an opaque or non-transparent container the time can be judged by observing the materials from the top and by experience. Substantially complete stratification should be allowed to result as it is not desired to have any appreciable quantity of free chloride in the dope nor to waste any material part of the dope.

After the treatment with the chloride it will be found that the viscosity of the dope originally added has been markedly increased so that whereas the dope was previously in the form of a viscous but readily fiowable solution it is now much thicker and not readily fiowable, being more in the nature of a gel.

After the stratification has become complete or substantially complete, the bottom outlet in the container is opened and the chloride is drained off, care being taken to drain off as much chloride as possible without wasting the treated dope. Generally, in accordance with best practice, all the chloride is drained out and a very small amount of the treated dope, the object being to remove completely the free chloride from the dope.

If desired, the dope may be employed as thus produced and prepared, although I generally thin it, as will be hereinafter pointed out. In most cases, however, I have discovered that it is desirable to wash the treated dope with plain water. The washing is carried out in the same way that the'do-pe was treated with chloride, i. e., to the same vessel with its bottom outlet closed, water is added to the treated dope and is thoroughly mixed and agitated therewith, following which the mixture is allowed to settle out and stratify. The water is then drawn off in the same manner as the chloride. Treatment with water ensures that no free chloride or acid shall remain in the treated dope.

The dope, whether it has been washed or not, is preferably thinned to give it a consistency at which it can be appropriately brushed onto the desired material or surfaces. To accomplish this thinning I add a suitable organic acetate, such as butyl, ethyl, methyl or amyl acetate, butyl acetate being preferred, and when employed is utilized in such proportion that it constitutes about 25% of the final material. This gives the dope a consistency similar to that of ordinary paint, thus lending itself well to brushing operations.

Before applying the treated dope I add thereto a suitable plasticizer for those applications in which a plasticizer is needed. This is necessary or desirable only when the dope has been washed, otherwise no plasticizer need be added. I may, for example, employ castor oil in the proportion of 1 ounce to each gallon of treated dope, but other plasticizers may also be employed.

Under some conditions I have discovered that for certain uses it is not necessary to have a dope which is non-inflammable since for those uses subjection to flame is unlikely and relatively remote. For such purposes I prefer to add to the treated dope, as above prepared, a proportion of untreated dope, the two being thoroughly mixed to provide a uniform product. The extent of resistance to fire of such a modified form of material is dependent upon the proportions of treated and untreated dopes present. For example, a mixture of about 50% treated dope and 50% untreated dope produces a material which is still fire-resistant, but various other proportions can be employed depending upon the intended use of the material, especially with relation to its probability of being subjected to burning conditions. I may also add commercial rubber cement to the treated dope in the proportion of about four (4) ounces thereof per gallon of fire-proofed dope. These ingredients are compatible and the resultant composition is perfectly sweat-resistant without adversely afiect'ing the tautening power, flexibility or tensile strength.

A non-inflammable dope as prepared above and without the addition of untreated dope is, I have found, a new and highly useful product, especially for application to various parts of airplanes. The treatment makes the skin or film derived from my new dope of substantially greater durability, particularly in the presence of strong sunlight, which has a marked tendency to deteriorate prior dopes and to cause them to sweat. The skin or film resulting from my new dope has undiminished tensile strength, the same tautening power and is further characterized by retaining its flexibility over an increased period of time. While I am not certain as to the chemical explanation as to precisely what occurs during agitation of the dope with zinc chloride, I believe that a chemical modification thereof takes place, probably accomplished by a catalytic effect. The fact that the treated dope is much more viscous and fuller bodied than the original untreated dope tends to confirm the foregoing. Rigorous tests carried out in connection with the treated dope clearly demonstrate its non-inflammable character and in the presence of different types of flames merely blackens or chars somewhat without burning or supporting combustion and without any extending of the blackened area to other areas. Thus it will be perceived that I have produced a material which is capable of securing the results set forth and at a very moderate cost. It will be understood that after the dope has been brushed onto the desired surface and the inner or solvent evaporated, a coat of varnish is applied over the dope, thus producing an excellent surface.

The foregoing is intended as illustrative and not as limitative, and within the scope and principles hereof I may resort to other and further additions, omissions, modifications and substitutions. While the invention has been particularly described in connection with the use of metallic zinc, Zinc oxide may also be used as above indicated. The only difierence is that in using zinc oxide the products of reaction are zinc chloride and water, whereas when metallic zinc is employed the products of reaction are zinc chloride and hydrogen. While I have particularly described my invention in connection with the use of nitrocellulose dope, it is to be understood that within this term are intended to be included the various compositions and formulae which are commonly produced, sold and used under the name of nitrocellulose dope or cellulose nitrate. While I have particularly described the invention in connection with the use of an acetate thinner, other thinners may nevertheless be employed.

QEGI'EH Gilli! Likewise, other plasticizers than castor oil may be introduced such as tricresyl phosphate. The invention is likewise applicable to other cellulose or cellulose nitrate compounds where such are inflammable and it is desired to render them noninflammable o-r fire-resistant.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A method of treating an inflammable nitrocellulose dope to render the same non-inflammable when dry, which comprises mixing and agitating inflammable nitrocellulose dope and zinc chloride in the proportion of about /2 gallon of nitrocellulose dope to an amount of Zinc chloride such as is obtained from reacting about 2 pounds of zinc with one gallon of commercial strength hydrochloric acid, settling the mixture of nitrocellulose dope and zinc chloride until there is a substantially sharp stratification between the dope and the chloride, removing the chloride and recovering the dope.

2. A method of treating an inflammable nitrocellulose dope to render the same non-inflammable when dry, which comprises mixing and agitating inflammable nitrocellulose dope and zinc chloride, at about room temperature in the proportions of about /2 gallon of nitrocellulose dope to an amount of zinc chloride such as is obtained from reacting about 2 pounds of zinc with one gallon of commercial strength hydrochloric acid, settling the mixture of nitrocellulose dope and zinc chloride to produce an upper layer of treated dope and a lower layer of chloride, separating the two layers, washing the dope layer with water and then separating the washed dope from such water and thinning the washed dope with an acetate from the group consisting of butyl acetate, amyl acetate, ethyl acetate, methyl acetate.

3. A method of treating an inflammable nitrocellulose dope to render the same non-inflammable when dry, which comprises mixing and agitating inflammable nitrocellulose dope and zinc chloride, at about room temperature in the proportions of about /2 gallon of nitrocellulose dope to an amount of zinc chloride such as is obtained from reacting about 2 pounds of zinc with one gallon of commercial strength hydrochloric acid, settling the mixture of nitrocellulose dope and zinc chloride to produce an upper layer of treated dope and a lower layer of chloride, separating the two layers, washing the dope layer with water and then separating the washed dope from such water, adding about 25% butyl acetate and a plasticizer such as castor oil in the proportion of one ounce of castor oil to each gallon of dope.

LINTON C. AMBERSON.

CERTIFICATE. OF CORRECTION.

Patent No. 2,569,200. February 15, 191 5.

LINTON c. AMBERSQN.

It is hereby'certified that error appears in the printed specification of the above mmibered -patent requiring correction as follows: Page 5 first column, line 56, for the word "inner" read -thinner--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of May, A. D. 1915.

Leslie Frazer (Seal) Acting Commissioner of Patents.

It is *herhj ror appears in of the bove nunibered pat ent requiring correction as fuiiows: Page 5; first I colum n; line 36, for the word "inner" read ---thinner---.; and that the ma Letters Patentshould be read with this correction therin that the .same may confom to the r cora of, the jcas e" in the Patent Office.

fiigned "and ,sehle'd' thi s-l5th d a y of'May-{A D 19 1-5;

s rZ f (Seal) Acting commis siojner bf Patents. 

